Delusion (1981)
Mostly forgotten, Delusion was picked up for distribution by Samuel Goldwyn and at first not necessarily advertised as a "slasher" rather a psychological drama mystery. It was later re-released theatrically with a more straight forward horror themed poster art, and at some point it received a title change in some markets as The House Where Death Lives. They went back to the original title Delusion for its home video debut.
I have really forgotten about this movie until I read that Vinegar Syndrome just released this to Blu-ray. Their review is pretty much spot on from what I remember. It is an extreme slow burner and for the most part doesn't feel like horror or a slasher at all. The makers were obviously trying a different approach, or maybe they didn't even had a slasher in mind when they made Delusion. The acting is fine, but the film is still not that good. Not your typical slasher flick. I'd like to rewatch this again, though.